Ultra low density polyolefin foam, foamable polyolefin compositions and process for making same

ABSTRACT

A foamable composition for producing an ultra low density polyolefin foam having a density of about 0.6 to about 1.5 pounds per cubic foot comprising a heat-plastified mixture comprising (1) an olefin polymer resin selected from the group consisting of ethylene homopolymers and copolymers of ethylene and a copolymerizable monomer; (2) an elastomer in an amount of about 3 to about 30 wt. parts per 100 wt. parts of said olefin polymer resin; (3) polystyrene in an amount of about 1 to about 15 wt. parts per 100 wt. parts of said olefin polymer resin; (4) a stability control agent selected from the group consisting of partial esters of long chain fatty acids with polyols, higher alkyl amines, fatty acid amides, and olefinically unsaturated carboxylic acid copolymers, and (5) a hydrocarbon blowing agent having from 1 to 6 carbon atoms and a boiling point between -175° C. and 50° C., said heat-plastified mixture being at a temperature and a pressure which does not allow said mixture to expand, said low density polyolefin foam made from said foamable composition and process of making said foam.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains to the art of making foamed low densitypolyethylene plastic of relatively large thickness and relatively largelateral cross-section and having a density of about 0.6 to about 1.5pounds per cubic foot (pcf). More specifically, this invention pertainsto the art of making ultra low density foamed plastic boards, planks,beams and the like of substantial dimensions measuring several inchesthick and wide and a few to several feet long used primarily forcushioning, packaging, building insulation, flotation, sound deadeningand other utilities.

2. Description of the Prior Art

Heretofore, polyolefin foamed plastics in the form of planks, beams,etc. were produced using a variety of blowing agents. For example, ithas been common practice to use chlorofluorocarbons (CFC's) and, morerecently, hydrochlorofluorocarbons (HCFC's) as blowing agent. Suchblowing agents have been found to deplete the planet Earth's ozone layerwhich serves as a shield to protect the planet from cancer-promotingultraviolet radiation. Governmental pressure is mounting to severelyrestrict the release of CFC's and HCFC's to the atmosphere in order toslow down or stop the depletion of the ozone layer.

There is great environmental and commercial interest in the eliminationof all ozone-depleting chemicals used as blowing agents. While HCFC'shave a lower ozone depletion potential (ODP) than CFC's, the level isstill unacceptably high and therefore the use of HCFC's is also comingunder greater regulation. It is possible to make polyolefin foam usinglight hydrocarbons such as isobutane which, however, leads to aworsening of physical properties, e.g., a decrease in thermal stability.

U.S. Pat. No. 3,067,147 (Rubens et al) discloses the production ofpolyethylene foam having a density as low as 1.7 pcf using1,2-dichloro-tetrafluoroethane. There is no disclosure of isobutane as ablowing agent or of any stability control agent. This patent disclosesthat other hydrocarbon blowing agents produce foams having non-uniformlarge cells.

U.S. Pat. No. 3,379,802 (Raley et al) and U.S. Pat. No. 3,766,099 (Kawaiet al) disclose polyethylene foams containing synthetic elastomers butfail to disclose polyolefin foams having densities as low as about 0.06pcf. The lowest density foam disclosed by Raley et al is 1.7 pcf and thelowest density foam disclosed by Kawai et al is 1.68 pcf. Kawai et alfails to utilize polystyrene or a stability control agent or anisobutane blowing agent and Raley et al fails to disclose theutilization of polystyrene and a stability control agent.

U.S. Pat. Nos. 4,370,378 and 4,387,169 (Zabrocki et al) disclosepolyethylene foams alleged to have densities of 1.1 to 1.88 pcf. TheZabrocki et al patents assert that the low densities are achievable byspecial mixing procedures. There is no disclosure in these patents ofusing an elastomer, polystyrene, a stability control agent or isobutane.To the contrary, the patents urge the use of ozone-depleting blowingagents such as the CFC's.

U.S. Pat. Nos. 4,640,933, 4,663,361 and 4,694,027 (Park) disclosepolyolefin foams having densities between 2.19 pcf and 2.66 pcf but failto disclose polyolefin foams of lower densities or the use of asynthetic elastomer or the achievement of densities as low as about 0.6pcf.

U.S. Pat. No. 4,652,590 (Hoki et al) discloses the manufacture of lowdensity polyethylene foams using CFC blowing agents. The patent fails todisclose the use of a synthetic elastomer or a stability control agentor isobutane as blowing agent.

U.S. Pat. Nos. 3,893,957, 4,721,591, 4,738,810 and 4,824,720 eachdisclose the manufacture of polyolefin foams but fail to disclose theuse of isobutane or other hydrocarbon blowing agent or the use ofstability control agents.

U.S. Pat. No. 4,379,859 (Hirosawa et al) discloses prefoamed particlesof polypropylene produced by dispersing the resin particles in waterfollowed by release into the atmosphere and drying.

SUMMARY OF THE INVENTION

An ultra low density polyethylene foam is desirable as cushioningmaterial due to its greater economy over standard density (2.2 pcf)materials. Previously, the lowest density commercialized foam ofrelatively large cross-sectional area has had a density of approximately1.4 pcf. It is possible to take polyethylene and add enough blowingagent to get the density down to approximately 1.2 pcf but the resultingfoam develops surface corrugations, ripples, warping or otherundesirable characteristics. Any attempt to add more blowing agentbeyond this point leads to a foam that does not have sufficient meltstrength to support its own weight, i.e., the foam collapses on itselfafter extrusion. The addition of an elastomer and polystyrene givessufficient elasticity and melt strength to permit the addition of enoughblowing agent so that densities of 0.6 pcf can be achieved.

As can be seen from a review of the above-noted prior art, althoughproviding many beneficial properties, the previously-known foamedplastics have failed to provide a foamed material with densities as lowas about 0.6 pcf which is produced with hydrocarbon blowing agents(e.g., butane or isobutane) that do not deplete the ozone layer in theupper atmosphere. By mixing low density polyethylene (hereinafter"LDPE"), an elastomer, such as, styrene-butadiene rubber (hereinafter"SBR"), and polystyrene (hereinafter "PS") with an appropriatehydrocarbon blowing agent, the present inventors have found that it ispossible to make, on a production basis, a foamed article of largecross-sectional area with a much lower density than has been previouslyachieved on a production basis. The foams of this invention areuncrosslinked and are comprised of predominantly closed cells having anaverage cell diameter of about 0.05 to about 0.1 inch, preferably about0.06 to about 0.07 inch. The densities of foams made according to thisinvention are consistently in the range of about 0.6 to about 1.5 pcf,preferably from about 0.7 to about 1.2 pcf. The foams produced accordingto this invention are substantially free of surface corrugations,ripples, warping, gas spots, and other undesirable characteristics.

This invention relates to the production of elongated, ultra low densitythermoplastic cellular bodies which can be several inches wide or, fromless than one inch to several inches thick, and up to several feet longfor use in such applications as building insulation purposes, flotationor buoyancy applications, packaging and for such diverse other uses aslarge art forms, floating pool toys, oil spill flotation containmentgear and the like. The present invention provides solutions to theproblems described hereinabove in relation to the prior art and providesa relatively low cost means for producing quantities of large size,ultra low density thermoplastic cellular products with a minimum ofcapital expenditure.

It is therefore a principal object of this invention to provide for theproduction of ultra low density, large size thermoplastic polymer foams.

It is another object to provide for the production of large size, ultralow density thermoplastic foams capable of periodic high rates ofproduction such as 5,000 or 12,000 or more pounds per hour of highquality, uniformly dimensioned, unwarped, large size, ultra low densitythermoplastic foam having a density as low as about 0.6 pcf.

It is a further object of this invention to provide means whereby largesize, low density thermoplastic polymer foams can be produced havingsubstantially uniform structural characteristics from end to end andbeing substantially free of non-uniform areas which require trimmingaway waste such as result from corrugations or ripples on the foam orwarping of the foam.

The present invention solves the prior art problems and achieves theobjects as disclosed hereinabove by the steps of forming a mixture of athermoplastic polymer, a synthetic elastomer, polystyrene, a stabilitycontrol agent and a hydrocarbon blowing agent and cooling the resultingfoamable mixture to a temperature at which the viscosity of the mixtureis adequate to retain the blowing agent when the mixture is subjected tolower pressure and is allowed to expand. After cooling, the foamablemixture is extruded into a holding zone maintained at a temperature andpressure that prevents foaming in the holding zone. The holding zone hasan outlet die having an orifice opening into a zone of lower pressuresuch as atmospheric pressure at which said foamable mixture foams andmeans for closing said orifice, said means being openable to allow thefoamable mixture to be ejected from said holding zone. In addition, amovable ram forces the foamable mixture out of the holding zone throughsaid die orifice at a rate greater than that at which substantialfoaming in the die orifice occurs and less than that at which meltfracture occurs, i.e., less than that at which substantialirregularities in cross-sectional area or shape of cellular body beingformed occurs. Upon passing through the die orifice into the zone oflower pressure, the foamable mixture is allowed to expand unrestrainedin at least one dimension to produce the desired large size, ultra lowdensity thermoplastic foam having a density as low as 0.6 pcf and beingrelatively free of surface corrugations, ripples, warping or substantialcell collapse.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Thermoplastic polymers usable in the present invention include olefinpolymer resins such as high, medium and low density, polyethylene,polypropylene, polyvinylchloride, and any other thermoplastic suitablefor use in manufacturing thermoplastic foams. Polyolefins suitable foruse in the practice of the present invention include ethylenehomopolymers such as low, medium, or high density polyethylene, andethylene copolymers such as ethylene-vinyl acetate copolymers,ethylene-1-butene copolymers, ethylene-butadiene copolymers,ethylene-vinyl chloride copolymers, ethylene-methyl methacrylatecopolymers, ethylene-acrylonitrile copolymers, ethylene-acrylic acidcopolymers, and the like. As the olefin polymer resin, it is preferableto use an ethylene homopolymer or a copolymer having an ethylene contentof at least about 50 percent by weight, preferably above 75 percent byweight. Such additional polymers are well known in the prior art and aredisclosed in the above-mentioned prior art patents and such disclosuresare incorporated herein by reference.

Suitable elastomers for use in this invention include randomstyrene-butadiene rubber, natural rubber, butadiene rubber, isobutylenerubber, ethylene-propylene rubber (EPR), diene-modifiedethylene-propylene rubber (EPDM) containing bound diene units derivedfrom 1,4-hexadiene, dicyclopentadiene, ethylidene norbornene, and thelike, acrylonitrile rubber, acrylonitrile-butadiene copolymer rubber,styrene-butadiene block copolymer rubber, poly(2-chlorobutadiene-1,3),synthetic polyisoprene, chlorinated copolymers of isobutylene,chlorosulfonated polyethylene and the like. Diene elastomers, that is,synthetic elastomers derived from dienes, are preferred. An example of asuitable commercially available elastomer is Stereon 840A, astyrene-butadiene block copolymer sold by Firestone Synthetic Rubber &Latex Company.

Any solid polystyrene is useful in this invention. For example, moldinggrade polystyrene such as Mobil PS 2520 easy flow crystal molding gradepolystyrene or Huntsman 206 polystyrene sold by Huntsman ChemicalCompany can be used.

Stability control agents suitable for use in the present inventioninclude the partial esters of long-chain fatty acids with polyolsdescribed in U.S. Pat. No. 3,644,230, as well as higher alkyl amines,fatty acid amides and complete esters of higher fatty acids such asthose described in Watanabe et al, U.S. Pat. No. 4,214,054 incorporatedherein by reference. Kemamide (trademark) S-180 stearyl stearamidestability control agent is one example of a suitable fatty acid amidestability control agent. Kemamide S-180 is commercially available fromHumko Chemical Division of Witco Chemical Corp. The partial esters offatty acids which are used in this invention are members of a genericclass known as surface active agents or surfactants. Exemplarysurfactants in the class of useful additives include, for example,glyceryl monostearate, glyceryl distearate, mixtures of these mono- anddiglycerides, glyceryl monobenzoate, sorbitan mono-, di-, andtrioleates, and mono- and diglycerides of oleic acid and palmitic acid,inter alia. Pationic 1052 sold by Patco Polymer Additives Division ofAmerican Ingredients Company and Atmos 150 sold by the Atlas PowderCompany are examples of suitable glyceryl monostearates that arecommercially available. Each of these products contain predominantlyglyceryl monostearate which may also contain some very small amounts ofglyceryl distearate. For ease of handling, the stability control agentis conveniently blended into the olefin polymer, e.g., LDPE, which isthen formed into pellets for feeding into the feed hopper of theextruder. In addition to, or in place of, the foregoing stabilitycontrol agents, there may also be employed for such purpose copolymersof α-olefins with various monoethylenically unsaturated carboxylic acidssuch as those described in Park, U.S. Pat. No. 4,347,329 or copolymersof α-olefins neutralized carboxyl-group bearing moieties which arecommonly referred to in the art as ionomers. Typically, sucholefinically unsaturated carboxylic acid copolymers may be employed inan amount ranging from about 5 to about 95% by weight of the olefinpolymer employed.

Isobutane is the most preferred blowing agent for use in this invention.However, other hydrocarbon blowing agents having 1 to 6 carbon atoms anda boiling point of -175° C. to 50° C. can be used. For example, suitablehydrocarbon blowing agents include n-propane, ethane, methane,propylene, n-butane, butylene, n-pentane, neopentane, 2-methylbutane,2,2-dimethylbutane, 2,3-dimethylbutane, n-hexane and the like.

In addition to the hydrocarbon blowing agent, the olefin polymeremployed in this invention can, and usually will, contain otheradditives for other purposes. For example, hydrocarbon polymers usuallycontain a small amount of a heat or light stabilizer and an antioxidantto prevent degradation during processing. Pigments, plasticizers,nucleating agents, wetting agents and mixing aids are also frequentlyemployed. The process is operable with any of such aids in the customaryquantities. For example, known nucleating (or cell-size controlling)agents include talc, clay, mica, diatomaceous earth, silica, titaniumoxide, zinc oxide, calcium silicate, metallic salts of fatty acids suchas barium stearate, zinc stearate, aluminum stearate, sodium bicarbonatewith or without citric acid, etc. and the like. The nucleating agentsare usually finely divided and are conveniently used as pellets madefrom about 20 wt. % nucleating agent blended into LDPE or other olefinpolymer.

The amount of elastomer used in the present invention can be varied inthe range of about 3 to about 30 wt. parts, preferably about 5 to about15 wt. parts, per 100 wt. parts of olefin polymer resin. The amount ofpolystyrene component used in this invention can be varied in the rangeof about 1 wt. part to about 15 wt. parts, preferably about 2 wt. partsto about 10 wt. parts, per 100 wt. parts of the olefin polymer resin.The amount of stability control agent used in this invention can bevaried from about 0.5 wt. part to about 10 wt. parts, preferably about 1wt. part to about 5 wt. parts per 100 wt. parts of olefin polymer resin.The amount of nucleating agent employed in the present invention can bevaried from about 0.02 wt. part to about 5 wt. parts, preferably about0.3 wt. part to about 3 wt. parts, per 100 wt. parts of olefin polymerresin. The amount of hydrocarbon blowing agent employed in thisinvention can be varied depending upon the density desired for theultimate foam. For example, from about 1 to about 50 wt. parts,preferably about 7 to about 40 wt. parts, of the blowing agent per 100wt. parts of the olefin polymer resin can be used.

In carrying out the method of this invention, the thermoplastic polymer,elastomer, polystyrene and other additives, usually in the form ofgranules, pellets or coarse powder are added to the extruder through thehopper and heated and masticated in the mixing zone of the extruder inthe usual way to produce a heat-plastified, molten mass of thermoplasticbeing mixed and advanced through the extruder. The stability controlagent is typically pre-blended into a portion of the thermoplastic andthe resulting pre-blend is formed into pellets which are fed into thefeed hopper with the major part of the thermoplastic granules, pelletsor coarse powder. The temperatures necessary for producing the moltenthermoplastic mass in the extruder are well known and fall in the rangeof 350° F. to 480° F., higher or lower, depending upon the particulartype of thermoplastic being used.

Preferably, at an intermediate point along the extruder, the hydrocarbonblowing agent which has a plasticizing effect, is pumped into the moltenthermoplastic mass and mixed therewith as the resulting mixture isadvanced through the extruder. The plasticizing action of the blowingagent enables cooling of the molten mixture of thermoplastic mass andblowing agent as it is forwarded in the forward end of the extruder.

The cooling of the molten foamable mixture of thermoplastic mass andblowing agent is important in order to enable the mixture to foam whenit is ejected into the zone of lower pressure to retain the blowingagent. This avoids loss of blowing agent and resultant collapse of thecellular structure due to the inability of the molten polymer to retainthe blowing agent within the cells formed by the expansion of theblowing agent. If the temperature of the foamable mixture ejected intothe lower (e.g., atmospheric) pressure zone is too high, thethermoplastic polymer portion of the mixture is too fluid, i.e., lackssufficient viscosity to retain the blowing agent within the mixture orcells formed by expansion of the blowing agent. The optimum temperaturerange to which the foamable mixture is cooled varies depending upon thetype of thermoplastic in the mixture and on other variables such as thetype and amount of blowing agent. For example, the optimum temperaturerange of the foamable mixtures leaving the extruder (for low densitypolyethylene) is about 180° F. to about 250° F., preferably about 210°F. to about 240° F., although higher or lower temperatures may beemployed.

The foamable mixture cooled to a temperature for example, in thepreferable range of about 210° F. to about 240° F. is introduced intothe holding zone of an accumulator such as that described in U.S. Pat.No. 4,323,528 (Collins) incorporated herein by reference. The holdingzone is maintained at a temperature (e.g., 180° F. to 240° F.) andpressure (e.g., 400 psig to 1,500 psig) which does not allow thefoamable mixture to foam. The holding zone is formed with an outlet diehaving an orifice which opens into a zone of lower pressure, forexample, the atmosphere. The die orifice is preferably externallyclosable by a gate which is movable externally of the holding zone toopen and close the die orifice. The movement of the gate does not, inany way, disturb or otherwise physically affect the foamable mixturewithin the holding zone other than to release it to the atmosphere whenopened.

The ejection rate, i.e., the time necessary to empty the holding chamberof the accumulator can be varied widely. Ejection rates of about 5,000to about 18,000 pounds per hour (pph), preferably about 8,000 pph toabout 12,000 pph can be used. Ejection rates are dependent on manyfactors such as the type of thermoplastic polymer being employed, thetype and amount of blowing agent employed, the amount of nucleation,i.e. nucleating agents, employed, the presence or absence of otherextrusion aiding additives, the temperature of the molten foamablemixture, the pressure under which it is stored in the holding chamber,the force and speed with which the ram is moved, and the size andconfiguration of the die orifice. The optimum rate of ejection toproduce the desired cellular body having the desired characteristics andsize can be readily arrived at for any particular composition of moltenfoamable mixture and any particular equipment by making a few runs andincreasing or decreasing the rate of ejection to produce the desiredcellular body.

The molten foamable mixture begins to expand as soon as it leaves thedie orifice of the accumulator and enters the zone of lower pressure,e.g., the atmosphere. The cellular body preferably is supported by meansof a conveyor system of some type, e.g., conveyor belt, or conveyorrollers, from the time that ejection is begun until ejection isterminated. The ejected foaming molten mixture continues to expandthroughout the entire ejection operation which normally takes from lessthan one second to several seconds and continues to expand even afterthe ejection operation has been completed. The expansion of the cellularbody continues for a few to several minutes after ejection is completedindicating the body is still deformable and when it is in an expandingor deformable condition it can be further shaped, for example, bytransfer molding or simply by altering one or more or all surfaces ofthe expanding cellular body. After a period of time the cellular bodyceases to expand any further which indicates that cooling has takenplace to the extent that the body is no longer totally deformable. Sincethe cellular body by its nature is a heat insulator, the internalportions remains hot and may remain deformable for a considerable periodof time after the outer areas have congealed and are no longerdeformable without the application of more heat.

While the hot cellular body is totally still in deformable condition, itcan be shaped by molding, for example, while it is still in the hotdeformable condition, the cellular body can be disposed between two moldhalves which are brought together to contact the outer surface of thecellular body. Because the cellular body is still expanding, it expandsinto contact with the mold surfaces which shape the body. As an example,surfing boards, can be produced from a flat or plank shaped cellularbody by bringing appropriately shaped mold halves together on the bodywhile the body is still expanding. Typically, ejection requires about 1second to about 10 seconds to be completed from opening of the dieorifice gate to closing the die orifice gate.

The thermoplastic cellular bodies produced by this invention are ofultra low density, of about 0.6 pcf to about 1.5 pcf, preferably ofabout 0.7 pcf to about 1.2 pcf. and, more preferably of about 0.9 pcf toabout 1.1 pcf. The cellular bodies produced by the present invention areof substantially consistent cross-section throughout their lengths. Thelengths of such bodies can be varied as desired from a few feet such as2,3 or 4 up to many feet, such as 12, 24, 48 or more feet depending uponthe size and capacity of the equipment used especially the size of thedie opening and the capacity of the holding chamber. In addition, thecellular bodies produced by this invention have a closed cellularstructure covered with a thin membrane and have substantially uniformultra low densities, cell size, K-factor and resiliency along the lengthof the body when such bodies are ejected and are allowed to expandfreely. Furthermore, the cellular bodies of this invention are capableof being produced with consistently uniform properties such as aredescribed above from run to run in commercial production equipment.

The cellular bodies produced by this invention can be in the form ofcylinders, planks, panels and can be formed with cross-sections that arecircular, flat, rectangular, arched or curved, right angled, square,triangular, S-shaped, T-shaped, X-shaped, or any other desirable shapeby selecting a die orifice capable of producing the desiredcross-sectional shape.

The cellular bodies of this invention are very light in weight and arehighly useful as cushioning materials in packaging delicate goods suchas computers, glassware, electronic equipment, such as TV sets,receivers, VCR's and the like, furniture, and any article that needs tobe protected from shock, gouging or surface-scratching or marring.Additionally, the cellular bodies of this invention find use in manyother applications as described in the above-mentioned Collins patent.

EXAMPLES 1 THROUGH 4 AND COMPARATIVE EXAMPLES A THROUGH E

In each of Examples 1 through 4, a low density polyethylene resin (LDPE)having a melt index of about 2 (ASTM D 1238) and a density of about0.922 (ASTM D 792) was blended with a styrene-butadiene rubber (SBR)having about 43 wt. % bound styrene, a melt index, (Cond. G, 200° C.,5000 g.) of 12 g./10 min. (ASTM D 1238), a specific gravity of 0.96 anda number average molecular weight, M_(n), of about 60,000 and a weightaverage molecular weight, M_(w), of about 85,000 (sold by FirestoneSynthetic Rubber & Latex Company under the designation STEREON 840A);polystyrene (PS) having a nominal melt flow rate (200° C., 5000 g.;Cond. G) of 12 g./10 min. and a Vicat softening temperature of 212° F.(ASTM D 1525); glyceryl monostearate (GMS) (Pationic 1052, a mixture ofglyceryl mono- and di-stearates, very predominantly glycerylmonostearate) as stability control agent; and diatomaceous earth (DE) asnucleating agent. The proportions of these ingredients are listed inTable I. In Examples A through E the proportions of LDPE, GMS and DE aslisted in Table I were blended. The resulting mixture in each ofExamples 1 through 4 and A through E was fed into the feed section of anextruder having a L:D ratio of about 48:1. The feed section temperaturewas maintained at 363° F. The mixture was then passed into the mixingzone of the extruder where the temperature was maintained at about 470°F. The respective amount of isobutane (IB) listed in Table I wasinjected into the mixture in the mixing zone in each case and theresulting mixture was then passed into a cooling zone maintained atabout 185° F. This temperature profile resulted in a melt temperature(of foamable plastic mass leaving the extruder) of about 220° F.

In each example the resulting foamable mixture was passed from theextruder into an accumulator such as that described in U.S. Pat. No.4,323,528. The accumulator was maintained at a temperature of about 210°F. and a pressure of about 1200 psi which were adequate to maintain themixture in foamable condition. When the desired amount of foamablemixture (about 300 cubic inches) filled the accumulator, the movable ramof the accumulator was actuated to eject the foamable mixture into theatmosphere at the rate of about 11,000 pounds per hour through a die toform a foamed plank approximately 2 inches×24 inches×216 inches.

In each example the resulting foam was analyzed for density in poundsper cubic foot, pcf, and the respective densities are set forth in TableI. In addition, the 25% compressive strength in pounds per square inch,psi, (pursuant to ASTM D 3575-84, Suffix D, paras. 17 through 23) andcompressive set in percent (pursuant to ASTM D 3575-84, Suffix B, paras.10 through 16) were measured and the results are given in Table I. Inaddition, the thermal stability was measured (pursuant to ASTM D3575-84, Suffix S, paras, 33 through 39) and the extent of change in thecross machine direction (CMD), in the machine direction (MD) and inthickness measured in percent are given for each example in Table I. Theminus sign indicates shrinkage. In addition, the sum of each of the CMD,MD and Thickness percentages of shrinkage is presented in each examplein Table I.

It is readily seen from the results given in Table I that the thermalstability of the foam produced pursuant to the present invention atdensities of about 0.68 pcf and 1.09 pcf are far superior to the thermalstability of foam produced at comparable densities made without theaddition of SBR or PS; compare Example 4 at 0.68 pcf with Example D atroughly 0.74 pcf in which the Example D "foamed" product had gas spotsand collapsed cells. Also, the foam of Example 4 at 0.68 pcf had a fargreater thermal stability than the foam of Example C at 0.95 pcf and thefoam of Example B at 1.09 pcf. These examples illustrate the superiorityof foamed products made pursuant to the present invention in the densityrange of about 0.6 to about 1.5 pcf.

                                      TABLE I                                     __________________________________________________________________________                    Example                                                                            Example                                                                            Example                                                                            Example                                                                            Example                                                                             Example                                                                            Example                                                                            Example                                                                            Example                              A    1    B    2    3     4    C    D    E                    __________________________________________________________________________    Composition                                                                   LDPE, wt. %     98.6 84.92                                                                              98.6 84.92                                                                              71.5  84.92                                                                              98.6 98.6 99.8                 SBR, wt. %      0    10   0    10   19.6  10   0    0    0                    PS, wt. %       0    3    0    3    6.0   3    0    0    0                    GMS, wt. %      1.2  2    1.2  2    2.5   2    1.2  1.2  0                    DE, wt. %       0.2  0.08 0.2  0.08 0.4   0.08 0.2  0.2  0.2                                  100  100  100  100  100   100  100  100  100                  IB, wt. % based on mixture                                                                    7    7.5  16.4 17.5 22.7  33.9 19.5 22   7                    Property                                                                      Density, pcf    2.15 2.13 1.09 0.99 0.74  0.68 0.95(1)                                                                            .sup.˜ 0.74(2)                                                               (3)                  25% Compressive Strength, psi                                                                 7.95 5.14 4.31 3.9  4.13  3.9  3.92                           Compressive Set, %                                                                            16.1 19.9 11.9 27.46                                                                              16.3  20.4 12.6                           Thermal Stability                                                             CMD, %          -3.5 -5.2 -9.8 -1.9 -7.6  -6.5 -14.2                          MD, %           -3.7 -7.6 -12.8                                                                              -3.8 -10.7 -6.5 -15.7                          Thickness, %    -9.7 -6.5 -8.4 -4.7 -10.3 -1.4 -11.1                          Sum             -16.9                                                                              -19.3                                                                              -31  -10.4                                                                              -28.6 -14.4                                                                              -41                            __________________________________________________________________________     (1) foam had corrugated surface, not suitable for production.                 (2) gas escaped forming gas spots, cells collapsed. Density given is only     a rough estimate because of presence of gas spots and cell collapse.          (3) foam collapsed.                                                      

What is claimed is:
 1. A process for producing a polyolefin foam havinga density of about 0.6 to about 1.5 pounds per cubic foot comprising (a)mixing and heat plastifying (1) an olefin polymer resin selected fromthe group consisting of low, medium, or high density polyethylene,ethylene-vinyl acetate copolymers, ethylene-1-butene copolymers,ethylene-butadiene copolymers, ethylene-vinyl chloride copolymers,ethylene-methyl methacrylate copolymers, ethylene-acrylonitrilecopolymers, and ethylene-acrylic acid copolymers, (2) an elastomerselected from the group consisting of random styrene-butadiene rubber,natural rubber, butadiene rubber, isobutylene rubber, ethylene-propylenerubber, diene-modified ethylene-propylene rubber containing bound dieneunits derived from 1,4-hexadiene, diene-modified ethylene-propylenerubber containing bound diene units derived from dicyclopentadiene,diene-modified ethylene-propylene rubber containing bound diene unitsderived from ethylidene norbornene, acrylonitrile rubber,acrylonitrile-butadiene copolymer rubber, styrene-butadiene blockcopolymer rubber, poly(2-chlorobutadiene-1,3), synthetic polyisoprene,chlorinated copolymers of isobutylene and chlorosulfonated polyethylenein an amount of about 3 to about 30 wt. parts per 100 wt. parts of saidolefin polymer resin, and (3) polystyrene in an amount of about 1 toabout 15 wt. parts per 100 wt. parts of said olefin polymer resin; (b)admixing the resulting heat plastified mixture with (4) a stabilitycontrol agent selected from the group consisting of partial esters oflong chain fatty acids with polyols, higher alkyl amines, fatty acidamides, and olefinically unsaturated carboxylic acid copolymers, and (5)a hydrocarbon blowing agent having from 1 to 6 carbon atoms and aboiling point between -175° C. and 50° C.; and (c) activating saidblowing agent to foam the resulting admixture to a substantially closedcell polyolefin foam having a density of about 0.6 to about 1.5 poundsper cubic foot.
 2. The process of claim 1 in which said olefin polymerresin is low density polyethylene.
 3. The process of claim 2 in whichsaid blowing agent is isobutane.
 4. The process of claim 3 in which saidstability control agent is glyceryl monostearate, glyceryl distearate ormixtures thereof.
 5. The process of claim 3 in which said elastomer is astyrene-butadiene rubber.
 6. The process of claim 5 in which saidstyrene-butadiene rubber is a block copolymer containing about 23 wt. %to about 75 wt. % of bound styrene.
 7. The process of claim 3 in whichthe amount of said elastomer is in the range of about 5 to about 15 wt.parts per 100 wt. parts of said olefin polymer resin.